(Reuters) -- United Nations human rights chief Navi Pillay on Monday condemned "appalling, widespread" crimes being committed by Islamic State forces in Iraq, including mass executions of prisoners and "ethnic and religious cleansing".

The persecution of entire communities and systematic violations by the al-Qaeda offshoot, documented by U.N. human rights investigators, would amount to crimes against humanity and war crimes under international law, she said in a statement.

"They are systematically targeting men, women and children based on their ethnic, religious or sectarian affiliation and are ruthlessly carrying out widespread ethnic and religious cleansing in the areas under their control."

Christians, Yazidis and Turkmen were among the minorities targeted by the Sunni militant group, which has forced people to convert to their strict form of Sharia law, she said.

Islamic State insurgents have captured a third of Iraq with little resistance and declared a caliphate in the areas of Iraq and Syria it controls. It has drawn the first American air strikes in Iraq since the end of the occupation in 2011.

Last week Islamic State released a video showing one of its fighters beheading the U.S. journalist James Foley, kidnapped in Syria in 2012. Their wealth and military might represent a major threat to the United States that may surpass that once posed by al Qaeda, the U.S. military says.

Some 1.2 million people have fled fighting and ISIL's advance in Iraq this year, the U.N. refugee agency says.

MASS EXECUTIONS, SLAVERY

The al-Qaeda splinter group seized control of the city of Mosul on June 10, in a spectacular show of strength against the Shi'ite-led Baghdad government.

ISIL loaded 1,000 to 1,500 prisoners from Badush prison in Mosul onto trucks and took them to a vacant area for screening, Pillay said. Sunni inmates were taken away again on the trucks.

"ISIL gunmen then yelled insults at the remaining prisoners, lined them up in four rows, ordered them to kneel and opened fire," she said.

Up to 670 prisoners from Badush prison were killed by Islamic State on June 10, she said, quoting dozens of survivors and witnesses, some of whom survived by pretending to be dead.

"Such cold-blooded, systematic and intentional killings of civilians, after singling them out for their religious affiliation, may amount to war crimes and crimes against humanity," said Pillay, a former U.N. war crimes judge who steps down on Aug. 31 after serving six years as U.N, rights boss.

In northern Nineveh province, hundreds of Yazidis were killed and up to 2,500 kidnapped in early August, Pillay said, citing testimony from victims and witnesses. Yazidis fled their ancient homeland of Sinjar and other villages to escape the militants, who regard the ethnic minority as devil worshippers.

Those who agreed to convert are being held by ISIL, but witnesses report that among those who refused, "men were executed while the women and their children were taken as slaves and either handed over to ISIL fighters as slaves or threatened with being sold", the U.N. statement said.

ISIL also killed and abducted hundreds of Yazidis in Cotcho village in southern Sinjar on Aug. 15, Pillay said, citing witness testimony including "harrowing phone calls".

U.N. human rights investigators have received increasing reports of civilians being targeted for killing, she said, citing incidents of dozens being killed in Basra and Diyala.

In Baghdad, medical sources indicate that at least 15 bodies are found in the city on a daily basis. "All appear to have been bound and executed", Pillay said.

Pillay called on the Iraqi government and international community to protect vulnerable ethnic and religious groups.

These included at least 13,000 Shia Turkmen in Salahuddin province besieged by ISIL forces since mid-June amid "fear of a possible, imminent massacre" and Yazidis in besieged villages of Sinjar who remain at "serious risk", she said.